T cells are important effector cells in natural antiviral and anticancer immunity. The ultimate goal of our studies is to reveal the cellular and molecular requirements for T cell differentiation and effector functions. This knowledge is needed for the development of novel strategies and drugs for immunomodulation. We explored the idea that the final outcome of antigen receptor- driven immune processes is at least partially determined by physiologically abundant small signaling molecules in extracellular environment of lymphocytes in different tissues. Extracellular purines (ATP and adenosine) and their (purinergic) receptors were studied as an example of such molecules. The results of our recent studies of adenosine receptors indicate that A2a receptors on T cell surface may play immunosuppressive role in conditions which lead to accumulation of extracellular adenosine. These conditions may include pharmacological intervention with widely used anti-inflammatory drugs (methotrexate and sulfasalazine) and extracellular environment near large solid tumors. Hypoxic conditions in such tumors are known to cause accumulation of extracellular adenonsine, which, in turn, could inhibit incoming anti-tumor cytotoxic T- lymphocytes from destroying the tumor. Such "defensive shield" of extracellular adenosine that large tumors create may be the reason of low success rate of immunoadoptive therapy of large (as contrasted with small sized) tumors. We expanded our earlier observations that low concentrations of extracellular adenosine strongly antagonized all known antigenic peptide or anti-TCR mAb-induced biochemical pathways of activation of T cells and their effector functions by providing evidence of extracellular mechanism of extracellular adenosine action on T-cells. The lymphoid cell lines from adenosine receptor and adenosine deaminase gene deficient mice are established to facilitate the molecular analysis of signaling by adenosine in regulation of immune response. Flow cytometry studies of patterns of A2A extracellular adenosine receptor expression in different functional subsets of human peripheral T cells were performed using unique reagent, anti- human recombinant A2A receptor mAb. The results of these studies demonstrated both the feasibility of using flow cytometry to quantify the expression of a G protein-coupled A2A receptor in different cell subsets and pointed to the possibility that while relatively high concentrations of extracellular adenosine inhibit T cell functions, the low "basal" levels of extracellular adenosine could be required for successful propagation of cytokine-secreting response by antigen-activated T cells. The definitive implication of adenosine receptors in these effects of adenosine may provide a novel molecular target for immunomodulation and for improving the anti-tumor T-killer cells response.